Chemical composition and method

ABSTRACT

The invention relates to a novel composition useful as a feed at production of chlorine dioxide, said composition being an aqueous solution comprising from about 1 to about 6.5 moles/liter of alkali metal chlorate, from about 1 to about 7 moles/liter of hydrogen peroxide and at least one of a protective colloid, a radical scavenger or a phosphonic acid based complexing agent, wherein the pH of the aqueous solution is from about 1 to about 4. The invention also concerns a process for producing chlorine dioxide using the novel composition

FIELD OF THE INVENTION

[0001] The present invention relates to a composition containing alkalimetal chlorate, hydrogen peroxide and at least one of a protectivecolloid, a radical scavenger or a phosphonic acid based complexingagent, and a process for producing chlorine dioxide using saidcomposition as a feed.

BACKGROUND OF THE INVENTION

[0002] Chlorine dioxide is primarily used in pulp bleaching, but thereis a growing interest of using it also in other applications such aswater purification, fat bleaching or removal of organic materials fromindustrial wastes. Since chlorine dioxide is not storage stable it mustbe produced on-site.

[0003] Production of chlorine dioxide in large scale is usuallyperformed by reacting alkali metal chlorate or chloric acid with areducing agent and recovering chlorine dioxide gas. Such processes aredescribed in, for example, U.S. Pat. Nos. 5,091,166, 5,091,167 and5,366,714, and EP patent 612686.

[0004] Production of chlorine dioxide in small scale, such as for waterpurification applications, can also be done from alkali metal chlorateand a reducing agent but requires somewhat different processes, such asthose described in U.S. Pat. 5,376,350 and 5,895,638.

[0005] The above small scale processes include feeding alkali metalchlorate, hydrogen peroxide and a mineral acid to a reactor, in whichchlorate ions are reduced to form chlorine dioxide. In these processesit has now been found favourable to use a premixed solution of alkalimetal chlorate and hydrogen peroxide as a feed. However, such solutionsare not storage stable, particularly due to decomposition of hydrogenperoxide, but there is also a risk for a reaction between the hydrogenperoxide and the chlorate to form chlorine dioxide. The decomposition ofhydrogen peroxide is particularly rapid in the presence of ferrous-and/or chromium ions, which may be introduced as in impurity in alkalimetal chlorate or be released from storage containers of steel.

SUMMARY OF THE INVENTION

[0006] It is an object of the present invention is to provide a storagestable aqueous mixture of alkali metal chlorate and hydrogen peroxidethat can be safely transported.

[0007] It is another object of the invention to provide a process forproducing chlorine dioxide, particularly in small scale, using such amixture as a feed.

[0008] It has surprisingly been found possible to meet these objects bya new composition being an aqueous solution comprising from about 1 toabout 6.5 moles/liter, preferably from about 3 to about 6 moles/liter ofalkali metal chlorate, from about 1 to about 7 moles/liter, preferablyfrom about 3 to about 5 mols/liter of hydrogen peroxide and at least oneof a protective colloid, a radical scavenger or a phosphonic acid basedcomplexing agent, wherein the pH of the aqueous solution is from about 1to about 4, preferably from about 1.5 to about 3.5, most preferably fromabout 2 to about 3. Preferably, at least one phosphonic acid basedcomplexing agents is present, preferably in an amount from about 0.1 toabout 5 mmoles/liter, most preferably from about 0.5 to about 3mmoles/liter. If a protective colloid is present, its concentration ispreferably from about 0.001 to about 0.5 moles/liter, most preferablyfrom about 0.02 to about 0.05 moles/liter. If a radical scavenger ispresent, its concentration is preferably from about 0.01 to about 1mol/liter, most preferably from about 0.02 to about 0.2 moles/liter. Thewater content in the composition is suitably from about 20 to about 70wt %, preferably from about 30 to about 60 wt %, most preferably fromabout 40 to about 55 wt %.

[0009] It has also surprisingly been found that the performance of asmall scale process for producing chlorine dioxide is improved is such acomposition is used as a feed. Thus, the invention also relates to apreferably continuous process for producing chlorine dioxide comprisingthe steps of:

[0010] (a) feeding an aqueous solution comprising alkali metal chlorate,hydrogen peroxide and at least one of a protective colloid, a radicalscavenger or a phosphonic acid based complexing agent as specified aboveand a mineral acid, or a mixture thereof, to a reactor to form anaqueous reaction mixture;

[0011] (b) reacting chlorate ions with hydrogen peroxide in saidreaction mixture to form chlorine dioxide; and

[0012] (c) recovering a product containing chlorine dioxide.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0013] As high pH favours decomposition of hydrogen peroxide, while lowpH favours formation of chlorine dioxide, it has unexpectedly been foundthat both can be avoided by selecting the above pH range. The pH isaffected, inter alia, by the amount of hydrogen peroxide and by theprotective colloid, radical scavenger or complexing agent used. Ifnecessary, the pH of the aqueous solution can be adjusted to a suitablelevel by adding small amounts of any acid or alkaline substancecompatible with hydrogen peroxide and chlorate, such as Na₄P₂O₇, orH₃PO₄.

[0014] Any phosphonic acid based complexing agent can be used.Particularly preferred compositions comprise at least one phosphonicacid based complexing agent selected from the group consisting of1-hydroxyethylidene-1,1-diphosphonic acid,1-aminoethane-1,1-diphosphonic acid, aminotri (methylenephosphonicacid), ethylene diamine tetra (methylenephosphonic acid), hexamethylenediamine tetra (methylenephosphonic acid), diethylenetriamine penta(methylenephosphonic acid), diethylenetriamine hexa (methylenephosphonicacid), and 1-aminoalkane-1,1-diphosphonic acids such asmorpholinomethane diphosphonic acid, N,N-dimethyl aminodimethyldiphosphonic acid, aminomethyl diphosphonic acid, or salts thereof,preferably sodium salts.

[0015] Useful protective colloids include tin compounds, such as alkalimetal stannate, particularly sodium stannate (Na₂(Sn(OH)₆). Usefulradical scavengers include pyridine carboxylic acids, such as2,6-pyridine dicarboxylic acid. It is to be understood that thecomposition of the invention can include mixtures of two or more of atleast one protective colloids, at least one radical scavenger and atleast one phosphonic acid based complexing agent.

[0016] In the aqueous solution of the new composition the molar ratioH₂O₂ to ClO₃ suitably is from about 0.2:1 to about 2:1, preferably fromabout 0.5:1 to about 1.5:1, most preferably from about from about 0.5:1to about 1:1. Using a composition of this ratio for producing chlorinedioxide has been found to give high conversion of the chlorate.

[0017] In order to inhibit corrosion, the composition further preferablycontains a nitrate salt, preferably alkali metal nitrate such as sodiumnitrate, in a preferred amount from about 1 to about 10 mmoles/liter,and a most preferred amount from about 4 to about 7 mmoles/liter.

[0018] It is also preferred that the amount of chloride ions is as lowas possible, preferably below about 0.5 mmoles/liter, most preferablybelow about 0.1 mmoles/liter, particularly below about 0.03mmoles/liter. Too much chloride increases the risk for corrosion, butmay also cause formation of chlorine when the composition is used forchlorine dioxide production. As chloride normally is present as animpurity in alkali metal chlorate, it is advisable to use chloratewithout extra added chloride, normally containing less than about 0.5,suitably less than about 0.05, preferably less than about 0.02, mostpreferably less than about 0.01 wt % of alkali metal chloride calculatedas NaCl in NaClO₃.

[0019] The composition normally contains as impurities ions of chromiumand iron, particularly Cr³⁺ and Fe²⁺. The presence of these ionsincreases the decomposition of the hydrogen peroxide, and it is desiredto keep their content as low as possible. However, they are inevitablyreleased during storage of the composition in steel containers, and mayalso be introduced as impurities in the alkali metal chlorate. Thecontent of Cr³⁺ is normally from about 0.5 to about 3 mg/liter,particularly from about 1 to about 2 mg/liter, while the content of Fe²⁺normally is from about 0.05 to about 5 mg/liter, particularly from about1 to about 2 mg/liter.

[0020] Any alkali metal chlorate can be used, such as sodium, potassiumor mixtures thereof, although sodium chlorate is preferred.

[0021] Besides the main ingredients discussed above and any unavoidableimpurities in the composition, it is preferred that the balance up to100% is mainly made up of water.

[0022] The novel composition may be prepared by simply mixing theingredients together, for example by dissolving solid alkali metalchlorate in water and adding aqueous solutions of hydrogen peroxide, andat least one of a protective colloid, a radical scavenger or acomplexing agent and any other optional substance. Alternatively, solidalkali metal chlorate may be dissolved in an aqueous solution ofhydrogen peroxide of suitable concentration, and adding the othercomponents before or after the alkali metal chlorate.

[0023] It has been found that a composition as described above issubstantially storage stable and can be transported safely. It is alsomore pleasant to handle for the plant operators as the content ofhydrogen peroxide is lower than in normal hydrogen peroxide of technicalgrade, which generally contains about 50 wt % H₂O₂.

[0024] In the process for producing chlorine dioxide of the invention, acomposition as described above and a mineral acid, preferably sulfuricacid, are used as feed materials. It has been found that when thecomposition of the invention is used as a feed, it is possible to avoidfeeding an unnecessary excess of water and thus obtaining a moreconcentrated reaction mixture and higher production. It has also beenfound that the consumption of the mineral acid is lower than if alkalimetal chlorate and hydrogen peroxide are fed separately, even if theyare pre-mixed before entering the reactor.

[0025] In the case sulfuric acid is used as a feed, it preferably has aconcentration from about 70 to about 96 wt %, most preferably from about75 to about 85 wt % and preferably a temperature from about 0 to about100° C., most preferably from about 20 to about 50° C., as it then maybe possible to operate the process adiabatically. Preferably from about2 to about 6 kg H₂SO₄, most preferably from about 3 to about 5 kg H₂SO₄is fed per kg ClO₂ produced. Alternatively, the equivalent amount ofanother mineral acid may be used.

[0026] A preferred process of the invention comprises the steps of:

[0027] (a) feeding a composition as described above and a mineral acid,or a mixture thereof, at one end of a tubular reactor to form a reactionmixture;

[0028] (b) reducing chlorate ions in the reaction mixture to in saidtubular reactor to form chlorine dioxide, wherein the degree of chlorateconversion to chlorine dioxide in said reactor suitably is from about75% to 100%, preferably from about 80 to 100%, most preferably fromabout 95 to 100%; and

[0029] (c) recovering a product containing chlorine dioxide at the otherend of said tubular reactor.

[0030] The product recovered is normally an aqueous solution containingchlorine dioxide, oxygen and an alkali metal salt of the mineral acid.It may also contains unreacted chemicals such as mineral acid and smallamounts of chlorate ions. However, it has been found possible to avoidany substantial formation of chlorine.

[0031] It is preferred to operate without recirculating unreactedchemicals such as chlorate or sulfuric acid from the product back to thereactor. In many applications the complete product mixture can be usedwithout separation, for example in water purification.

[0032] It is normally favourable to operate the reactor as a CFSTR(constant flow stirred tank reactor). The reaction mixture in the bulkof the reactor preferably contains from 0 to about 2, most preferablyfrom 0 to about 0.1 moles per liter of chlorate ions, and from about 3to about 10, most preferably from about 4 to about 6 moles per liter ofsulfuric acid. It is preferred to maintain the concentration of chlorateand sulfate below saturation to avoid crystallization of metal saltsthereof.

[0033] Suitably the pressure in the reactor is from about 17 to about120 kPa, preferably from about 47 to about 101 kPa, most preferably fromabout 67 to about 87 kPa. Although normally not necessary, it ispossible also to supply extra inert gas such as air. The temperature ispreferably maintained from about 30° C. to the boiling point of thereaction mixture, most preferably below the boiling point.

[0034] It is preferred that the composition of the invention issubstantially uniformly dispersed in the mineral acid at the inlet ofthe reactor to avoid any substantial radial concentration gradients overthe cross section of the reactor. In order to minimize the radialconcentration gradients it has been found favourable to use a tubularreactor with a inner diameter from about 25 to about 250 mm, preferablyfrom about 70 to about 130 mm.

[0035] The process of the invention is particularly suitably forproduction of chlorine dioxide in small scale, for example from about0.1 to about 100 kg/hr, preferably from about 0.1 to about 50 kg/hr inone reactor. For many applications, a suitable chlorine dioxideproduction rate is from about 0.1 to about 10 kg/hr, preferably fromabout 0.2 to about 7 kg/hr, most preferably from about 0.5 to about 5kg/hr in one reactor. It is possible to achieve a high degree ofchlorate conversion in a comparatively short reactor, preferably havinga length from about 50 to about 500 mm, most preferably from about 100to about 400 mm. It is particularly favourable to use a tubular reactorhaving a preferred ratio of the length to the inner diameter from about12:1 to about 1:1, most preferably from about 4:1 to about 1.5:1. Asuitable average residence time in the reactor is from about 1 to about100 minutes, preferably from about 4 to about 40 minutes.

[0036] A small scale production unit normally consist of only onereactor, but it is possible to arrange several, for example up to about15 or more reactors in parallel, for example as a bundle of tubes.

EXAMPLE 1

[0037] A process of the invention was run by continuously feeding 78 wt% H₂SO₄ and a composition according to the invention to a tubularreactor having an internal diameter of 100 mm and a length of 300 mm.The composition of the invention was an aqueous solution of 40 wt %NaClO₃, 10 wt % H₂O₂, and containing diethylene triamin pentaphosphonate(Dequest™ 2066A). The reactor was operated at a pressure of 500 mm Hg(67 kPa), a temperature of 40° C. and produced 5 lb (2.3 kg) ClO₂ perhr. As a comparison, a process was run in the same way, with theexception that instead of feeding a composition according to theinvention, aqueous solutions of 40 wt % NaClO₃ and of 50 wt % H₂O₂ werefed separately. As shown in the table below, it was found that byfeeding a composition according to the invention less sulfuric acid wasrequired to reach the same conversion degree of chlorate. Flow rate(ml/min) ClO₃ ⁻ Comp. of conversion invention NaClO₃ H₂O₂ H₂SO₄ (%) 12333 4,75 81 123 33 4,29 81 123 33 4,41 84 123 33 4,51 85 123 33 4,53 86123 33 4,77 87 123 33 5,26 90 123 33 4,61 91 123 33 4,55 92 123 33 4,9293 123 33 5,1 93 123 33 5,42 93 123 33 5,22 93 123 33 4,84 94 122 3,3 83122 3,31 86 122 3,42 87 122 3,48 89 122 4,45 90 122 3,8 90 122 3,68 90122 3,56 90 122 4,29 91 122 4,16 91 122 4,04 91 122 3,86 92 122 4,78 96

EXAMPLE 2

[0038] A composition according to the invention was prepared byproviding an aqueous solution of 40 wt % NaClO₃, about 10 wt % H₂O₂, and500 mg/l diethylene triamin pentaphosphonate (Dequest™ 2066A). The pHwas adjusted by adding Na₄P₂O₇. The prepared solutions contained asimpurities 2 mg/l Fe²⁺ and 2 mg/l Cr³⁺. Samples of the solutions werestored in vessels of passivated steel (SS 2343) at 55° C., and thedecomposition degree of the hydrogen peroxide was measured after 14days. For comparative purposes, compositions without diethylene triaminpentaphosphonate were stored in the same way. No chlorine dioxide wasformed in any of the samples, but as shown in the table below, thecomposition of the invention had satisfactory stability, while asubstantial amount of the hydrogen peroxide had decomposed in thecomparative composition. pH wt % H₂O₂ % H₂O₂ Decomp Exp. No. start startafter 14 days 1 (invention) 2,1 9,8  4 2 (invention) 3,2 9,9  5 3(comparative) 2,2 9,8 55 4 (comparative) 3,6 9,9 84

1. Composition useful as a feed at production of chlorine dioxide, saidcomposition being an aqueous solution comprising from about 1 to about6.5 moles/liter of alkali metal chlorate, from about 1 to about 7moles/liter of hydrogen peroxide and at least one of a protectivecolloid, a radical scavenger or a phosphonic acid based complexingagent, wherein the pH of the aqueous solution is from about 1 to about4.
 2. Composition as claimed in claim 1, wherein the aqueous solutioncomprises at least one phosphonic acid based complexing agent. 3.Composition as claimed in claim 1, wherein the aqueous solutioncomprises at least one protective colloid.
 4. Composition as claimed inclaim 1, wherein the aqueous solution comprises at least one radicalscavenger.
 5. Composition as claimed in claim 2, wherein the aqueoussolution comprises from about 0.1 to about 5 mmoles/liter of at leastone phosphonic acid based complexing agent.
 6. Composition as claimedclaim 2, wherein the at least one phosphonic acid based complexing agentis selected from the group consisting of1-hydroxyethylidene-1,1-diphosphonic acid,1-aminoethane-1,1-diphosphonic acid, aminotri (methylenephosphonicacid), ethylene diamine tetra (methylenephosphonic acid), hexamethylenediamine tetra (methylenephosphonic acid), diethylenetriamine penta(methylenephosphonic acid), diethylenetriamine hexa (methylenephosphonicacid), morpholinomethane diphosphonic acid, N,N-dimethyl aminodimethyldiphosphonic acid, aminomethyl diphosphonic acid, and salts thereof. 7.Composition as claimed claim 5, wherein the at least one phosphonic acidbased complexing agent is selected from the group consisting of1-hydroxyethylidene-1,1-diphosphonic acid,1-aminoethane-1,1-diphosphonic acid, aminotri (methylenephosphonicacid), ethylene diamine tetra (methylenephosphonic acid), hexamethylenediamine tetra (methylenephosphonic acid), diethylenetriamine penta(methylenephosphonic acid), diethylenetriamine hexa (methylenephosphonicacid), morpholinomethane diphosphonic acid, N,N-dimethyl aminodimethyldiphosphonic acid, aminomethyl diphosphonic acid, and salts thereof. 8.Composition as claimed in claim 1, wherein the water content is fromabout 20 to about 70 wt %.
 9. Composition as claimed in claim 8, whereinthe water content is from about 30 to about 60 wt %.
 10. Composition asclaimed in claim 1, wherein the aqueous solution comprises from about 1to about 10 mmoles liter of alkali metal nitrate.
 11. Composition asclaimed claim 1, wherein the content of chloride ions in the aqueoussolution is less than 0.5 mmoles/liter.
 12. Composition as claimed claim1, wherein the molar ratio of H₂O₂ to ClO₃ ⁻ in the aqueous solution isfrom about 0.2:1 to about 2:1.
 13. Composition as claimed claim 1,wherein the aqueous solution comprises from about 3 to about 6moles/liter of alkali metal chlorate, from about 3 to about 5moles/liter of hydrogen peroxide and from about 0.5 to about 3mmoles/liter of at least one phosphonic acid based complexing agent. 14.Composition as claimed claim 1, wherein the pH of the aqueous solutionis from about 1.5 to about 3.5.
 15. A process for producing chlorinedioxide comprising the steps of: (a) feeding an aqueous solutioncomprising alkali metal chlorate, hydrogen peroxide and at least one ofa protective colloid, a radical scavenger or a phosphonic acid basedcomplexing agent as defined in claim 1 and a mineral acid, or a mixturethereof, to a reactor to form an aqueous reaction mixture; (b) reactingchlorate ions with hydrogen peroxide in said reaction mixture to formchlorine dioxide; and (c) recovering a product containing chlorinedioxide.
 16. A process as claimed in claim 15, wherein said aqueoussolution in step (a) is a composition as defined in claim
 2. 17. Aprocess as claimed in claim 15, wherein said aqueous solution in step(a) is a composition as defined in claim
 3. 18. A process as claimed inclaim 15, wherein said aqueous solution in step (a) is a composition asdefined in claim
 4. 19. A process as claimed in claim 15, wherein saidaqueous solution in step (a) is a composition as defined in claim
 5. 20.A process as claimed in claim 15, wherein said aqueous solution in step(a) is a composition as defined in claim
 6. 21. A process as claimed inclaim 15, wherein said aqueous solution in step (a) is a composition asdefined in claim
 7. 22. A process as claimed in claim 15, wherein saidaqueous solution in step (a) is a composition as defined in claim
 8. 23.A process as claimed in claim 15, wherein said aqueous solution in step(a) is a composition as defined in claim
 9. 24. A process as claimed inclaim 15, wherein said aqueous solution in step (a) is a composition asdefined in claim
 10. 25. A process as claimed in claim 15, wherein saidaqueous solution in step (a) is a composition as defined in claim 11.26. A process as claimed in claim 15, wherein said aqueous solution instep (a) is a composition as defined in claim
 12. 27. A process asclaimed in claim 15, wherein said aqueous solution in step (a) is acomposition as defined in claim
 13. 28. A process as claimed in claim15, wherein said aqueous solution in step (a) is a composition asdefined in claim
 14. 29. A process as claimed in claim 15, wherein themineral acid is sulfuric acid.
 30. A process as claimed in claim 15,wherein the product recovered in step (c) is an aqueous solutioncontaining chlorine dioxide, oxygen and an alkali metal salt of themineral acid.
 31. A process for producing chlorine dioxide comprisingthe steps of: (a) feeding a composition as defined in claim 1 and amineral acid, or a mixture thereof, at one end of a tubular reactor toform a reaction mixture; (b) reducing chlorate ions in the reactionmixture to in said tubular reactor to form chlorine dioxide, wherein thedegree of chlorate conversion to chlorine dioxide in said reactor isfrom about 75 % to 100 %; and (c) recovering a product containingchlorine dioxide at the other end of said tubular reactor.